In general, it is preferable that engine torque (hereinafter, referred to as torque) of an internal combustion engine (hereinafter, referred to as an engine) be as constant (flat) as possible regardless of a speed. However, the torque of the engine is actually changed according to a speed. In the past, a torque increase resonator (hereinafter, referred to as a resonator) has been used to make the torque characteristics of the engine be as flat as possible. The resonator is an additional device that increases torque by improving the suction efficiency of air sucked into a combustion chamber through the amplification of pulsation of air (intake pulsation) that is generated as air flowing through an intake passage of the engine is sucked into the combustion chamber. The resonator amplifies intake pulsation by using Helmholtz resonance. When the frequency of intake pulsation (hereinafter, referred to as pulsation frequency) corresponds to the resonant frequency of the resonator, intake pulsation is increased and torque is increased.
Since pulsation frequency is changed according to an engine speed, the resonator is adapted so that resonance occurs at a desired engine speed (pulsation frequency) where torque is to be increased. However, as a frequency deviates from the resonant frequency in the resonator, torque is reduced. Accordingly, when the range of a speed where torque is to be increased is wide, a variable resonator switching a plurality of resonant frequencies is used.
JP 2011-241787A (Reference 1) discloses a resonator that is provided in a surge tank supplying intake air to an engine and includes a volume unit, a communication unit, and a resonant frequency switching unit. The communication unit connects the surge tank to the volume unit, and includes a communication passage that allows a tank chamber to communicate with a volume chamber. The resonant frequency switching unit switches a resonant frequency of the resonator into a first predetermined frequency and a second predetermined frequency by rotating a variable-volume partition plate according to an opening of a throttle and the speed of an engine by an actuator so as to change the volume of the volume chamber, the cross-sectional area of the communication passage, and the length of the communication passage.
JP 2001-123902A (Reference 2) discloses a resonator that includes a partition plate provided in a box body so as to be divided from a surge tank, a communication hole communicating with the surge tank and formed in the partition plate, and a movable plate including a plurality of communication pipes and rotated by an actuator. In the resonator, the movable plate is rotated according to drive conditions of an engine so that one of the plurality of communication pipes is matched to the communication hole. Accordingly, the surge tank and the resonator communicate with each other and a resonant frequency is changed.
Since the resonator has a function of amplifying intake pulsation in an intake manifold, a resonance pipe or a resonance chamber of the resonator requires strength that does not allow deformation even though strong pulsation is applied to the resonance pipe or the resonance chamber and high airtightness that retains pressure of intake pulsation. Further, in a variable resonator, for the significant change of a resonant frequency of the resonator, it is necessary to significantly change one of the volume of the resonance chamber, the length of the resonance pipe, and the diameter of the resonance pipe or to simultaneously change two or three of them.
In JP 2011-241787A (Reference 1), all of the volume of the volume chamber, the cross-sectional area of the communication passage, and the length of the communication passage are significantly changed when a first fan-shaped partition plate and a second fan-shaped partition plate are rotated in directions opposite to each other together with the rotation of the variable-volume partition plate. For this reason, there is a concern that the structure of the resonator becomes complicated and increases in size. Furthermore, since the actuator rotating the variable-volume partition plate is positioned outside the volume chamber, a shaft (a rotating shaft of the actuator) on which the variable-volume partition plate or the first and second fan-shaped partition plates are mounted is disposed so as to pass through the volume chamber from the outside toward the inside. For this reason, there has been a problem in that it is difficult to ensure high airtightness even though, for example, a seal is provided.
In JP 2001-123902A (Reference 2), the actuator is provided in the resonator. However, in order to obtain a desired resonant frequency, airtightness should be ensured between the movable plate and the communication hole and a flow passage of air between the surge tank and the resonator should be formed by only the communication hole and the communication pipe. For this purpose, although not specified in JP 2001-123902A (Reference 2), for example, a seal made of rubber or the like needs to be provided on the outer periphery of an opening of the communication hole facing the resonator and the movable plate needs to be rotated while coming into contact with the seal. For this reason, there have been problems in that a frictional force caused by the contact between the movable plate and the seal is always generated during the rotation of the movable plate and an actuator having a large output is needed to smoothly rotate the movable plate. Further, even though the actuator having a large output can be used, there has been a problem in that the seal wears out with the rotation of the movable plate and airtightness deteriorates.